PORTLAND, Ore. -- Nanomaterials are often cited as being up to a thousand times stronger than steel, but researchers have had a difficult time transferring that strength to bulk materials. Now, an engineering professor at the University of Michigan (Ann Arbor) claims to have invented a "brick-and-mortar" technique that achieves that goal by mimicking the way oysters embed calcium carbonate into an organic matrix to create sea-shells--one of the strongest materials found in nature. The result is a material as strong as steel, but ultra-thin and transparent.
"We set out to transfer the intrinsic strength of nanomaterials to a macroscopic material by embedding the nanomaterial in an [organic] polymer matrix," said engineering professor Nicholas Kotov. "This was very difficult, because when stress deforms or stretches the bulk material, it's the interface between the nanomaterial and the polymer that tends to fail."
In other words, even though nanomaterials have super-strength, that durability cannot be transferred to the bulk material if the interface with its polymer matrix fails prematurely. Oysters solve this problem by gluing together nanoscale layers of super-strength carbon with organic glue in a brick-and-mortar fashion. Just as the strength of bricks is transferred to a wall by the overlapping, staggering and gluing with mortar, Kotov uses alternating layers of polymer "mortar" with "bricks" of strong carbon-based materials.
"We have solved the interface problem, and successfully transferred the strength of the nanomaterial to the bulk material, with a new type of polymer glue--polyvinyl alcohol--and a new type of layer-by-layer assembly method that is very uniform," said Kotov.
Instead of using carbon nanotubes, Kotov's current material uses a phyllosilicate mineral (clay,) which has the strength of the pure carbon in a nanotube, but also has atoms of aluminum, oxygen and hydrogen in its crystalline lattice to facilitate the brick-and-mortar construction method.
"The way that the oxygen atoms are arranged around the silicon atoms is the same as the structure of carbon atoms in diamond--making it a very stable and robust structure," explained Kotov. "And the hydrogen atoms act like Velcro--if stress breaks one hydrogen bond, it can reform with an adjacent atom to maintain the overall strength of the material."
The bulk material created by Kotov has up to 300 individually deposited layers of the nanoscale material alternating with atomically thin layers of polyvinyl alcohol, resulting in a material that is stronger than steel but as thin as cellophane.
The successful transfer of the strength of the nanomaterials to an ultra-thin bulk material has enabled Kotov's research group to land contracts with the Office of Naval Research (ONR) for stronger-than-steel but ultra-light body armor. The Air Force Office of Scientific Research is also funding a project to create amazingly strong yet ultra-light wings for its unmanned aerial vehicles (UAVs). To serve commercial applications, Kotov has founded a company to develop and market the materials: Nico Technologies Corp. (Ann Arbor, Mich.).
"One of our most interesting applications is for microelectromechanical systems—MEMS," said Kotov. "We are also working on flow-control valves, biosensors, RFID circuits, and micro-actuators using our material."